A pressure sensitive adhesive, generally, is a material which adheres to a surface with slight pressure and releases from the surface with negligible transfer of the material to the surface. Silicone pressure sensitive adhesives that are known in the art are typically solvent-based adhesives; the solvents are employed primarily to reduce the silicone pressure sensitive adhesive's viscosity to a viscosity which is easily coated onto the substrate of choice, and the solvents are removed after coating. As with any solvent-based pressure sensitive adhesive (PSA), special precautions must be taken to contain and avoid environmental exposure of the solvents and avoid flammable and explosive conditions since many of the solvents used are flammable.
Hot-melt pressure sensitive adhesives are those adhesives, which upon heating, melt to viscosities suitable for coating, but when cooled are generally in a flowless state. The advantages of hot-melt PSA's relative to solvent-based PSA's are known. Namely, the advantages of hot-melt PSA's are that they (1) do not require removal and containment of solvents, (2) do not require special precautions to avoid fires due to the presence flammable solvents, (3) make available coating processes other than those commonly used with solvent-based PSA's and (4) are more easily coated into thick thicknesses with minimal bubbling which often results with solvent-containing PSA's. In addition, solventless PSA's have the advantage of not containing solvents which sometimes interfere with the addition of other ingredients to the PSA.
Silicone pressure sensitive adhesives have been found to be preferred over other types of PSA's in many applications, especially in the medical area. For example, silicone pressure sensitive adhesives, due to the fact that they are acceptable for topical use, have found use in transdermal drug delivery applications which involve the adherence of a drug-containing patch to a patient's skin.
U.S. Pat. No. 4,865,920 to Randall P. Sweet, also the inventor of this invention, discloses a method of making hot-melt silicone pressure sensitive adhesives which have the inherent benefits of being composed of silicone and being a hot-melt PSA. In U.S. Pat. No. 4,865,920, the hot-melt silicone pressure adhesive composition consists of (i) a silicone resin, (ii) a silicone fluid, and (iii) 1 to 10 weight percent, based on the total weight of the silicone resin and silicone fluid, of an ester having the formula: R--C(O)OR' wherein R is a monovalent non-flammable hydrocarbon radical having from 2 to 32 carbon atoms and R' is a monovalent non-flammable hydrocarbon radical having from 1 to 14 carbon atoms. Although this silicone pressure sensitive adhesive composition has been found to be highly efficacious, it is desirable to have a hot-melt silicone pressure sensitive adhesive which uses ingredients which are (1) less toxic and more skin-compatible, (2) more available, and (3) less expensive than the aforementioned esters. The higher degree of skin compatibility is desirable especially for dermal applications.
It is also desirable that the new hot-melt silicone pressure sensitive adhesive be capable of being substantially transparent and have controllable adhesion, so that the aggressiveness of adhesion can be tailored to the application. For example, in terms of the transdermal drug delivery patch application, it is desired that the PSA exhibit a suitable adherence to the skin so that it remains adhered for the desired amount of time, but is comfortable to the patient upon removal.